Doc Brown's Chemistry

The pH scale of acidity and alkalinity, acids, alkalis, salts and neutralisation

1. Examples of everyday acid-alkali chemistry

Revision Notes KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  (revise courses equal to US grades 9-10)

Advanced Level Chemistry Acid-Base Revision Notes - use index

GCSE Sub-index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of acid-alkali chemistry : 2. pH scale, indicators, ionic theory of acids-alkali neutralisation : 3. pH examples of acid, neutral or alkaline solutions : 4. Acid reactions with metals/oxides/hydroxides/carbonates and neutralisation reactions : 5. Reactions of bases-alkalis like sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation : 8. Important formulae, salt solubility and water of crystallisation : 9. Further examples of word/symbol equations for salt preparations : 10. More on Acid-Base Theory and Weak and Strong Acids : EMAIL query?comment

In the HOME: Alkaline lime (CaO, calcium oxide), is put on soil that is too acid for healthy plant growth. Powdered limestone (CaCO₃, calcium carbonate) is slower and less effective. Both react and neutralise acids and you can pre-test the soil with pH paper. They can be used on a larger scale in farming and rivers and lakes.

Antacid indigestion tablets are mild alkalis that react by neutralising excess stomach acid which is the 'strong' hydrochloric acid which your delicate stomach lining and upper gut can only take so much of! The antacids must be weak (i.e. mild) alkalis or strong alkalis can be just as irritating as strong acids!

Bicarbonate or (sodium hydrogencarbonate NaHCO₃, baking powder) can be used with sour milk (acidic) for raising action in baking. The acidic milk reacts with 'Bicarb' to form carbon dioxide gas giving the rising action.

Acidic bee stings (pH 5.0-5.5) can be soothed, i.e. neutralised by calomine lotion, which is a mild alkali and antiseptic and anti-itching agent based on zinc oxide. You can also use baking soda ('bicarb of soda' or sodium hydrogen carbonate), another mild alkali. Wasp stings are supposed to be alkaline, but apparently not so! they are almost neutral at pH 6.8-6.9 but are 'traditionally' treated with vinegar which is a weak acid (and then perhaps you need the calomine too!). I've come across references on the web to say that wasp stings are not alkaline so 'English folklore' and mild-weak acid treatment has no real scientific basis. It should be pointed out that sting venom is a complex mixture, including many protein-enzymes, which, with other 'foreign' substances, might well trigger a response from the bodies immune system, so, in all honesty, I'm not quite sure what the truth is! However, what is known is that (i) bees and wasps have glands that can secrete either acids or alkalis with other substances and (ii) ants sting venom often contains methanoic acid ('formic acid') which can have a pH of 3 and is presumably 'soothed' by mild alkalis and just to confuse matters more, (iii) many people claim the 'folklore' remedies work! and maybe they do!

In the chemical INDUSTRY

Sodium hydroxide, one of the most commonly used alkalis, is used to neutralise aspirin making 'soluble aspirin'. Aspirin is an organic acid and not very soluble in water, its sodium salt is much more soluble and is absorbed faster by the body.

Ammonia gas is a weak alkali and neutralised by sulphuric acid or nitric acid to form ammonium sulphate or ammonium nitrate salts. These are important agri-chemical fertilisers supplying nitrogen to the soil for better plant growth. Of course some people prefer organic growing using good old muck and compost, but it doesn't involve neutralisation, but it does involve my wife, who is a member of the Soil Association!

Neutralising harmful sulphur dioxide gas (acidic, irritating and toxic SO₂) in power station smoke from burning fossil fuels, by absorbing it in alkaline calcium hydroxide solution (limewater) to absorb it. Eventually harmless calcium sulphate solution is formed.

Acids can be used to clean corroded metal surfaces because of their reactivity to metals and metal oxides to form soluble salts which can be washed away to leave a cleaner metal surface.

So all of this is still pretty important chemistry even for the 21st century, with strong links to agriculture, the environment and leading a stressful life!

Of course there are 'downsides' to some of this 'acidic' chemistry: Acid rain increases the rate of corrosion of stonework (particularly limestone) and metal structures. Acid rain makes water too acid for some aquatic organisms to live and this in turn affects food chains e.g. salmon do not like water with a pH below 4.5! Living on Venus could be hard going, its atmosphere is 98% sulphuric acid, mind you, you should be ok in a plastic suit because plastics don't usually react with acids, which is why, as well as being cheaper, plastics are replacing water pipes, drain pipes and gutters etc.

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